The present invention is directed to a monitoring apparatus for continuous physical integrity monitoring of large civil structures, such as bridges and high-rise buildings, wherein relevant censored data is generated continuously and transmitted to a data gathering location. Specifically, the present invention is an improvement in the optical encoder system and the corrosion monitoring system which are shown and described in my co-pending Patent Cooperation Treaty Application, PCT/US96/20015 filed Dec. 13, 1996 and entitled xe2x80x9cStructural Monitoring Sensor Systemxe2x80x9d. In this co-pending application, an optical structural integrity monitoring system includes a sensor interrogation harness which exploits a sensor differential technique known as TDM (time-division multi-plexing). Since light travels through an optical fiber at a fixed velocity, each sensor is attached to the pulse laser source by a different length of fiber. Further, by also causing the sensors"" output to be reflected back down the same fiber to the photo detector, the differential delay is precisely doubled.
The optical monitoring system of my co-pending application includes a laser or other semi-conductor light source which is capable of generating pulses of light into one leg of a Y-coupler. The other leg of the coupler is connected to a photo detector which, in turn, is operatively connected to circuitry. A cabled bundle of optical fibers is connected to the Y-coupler. A single optical fiber from the cable is connected to each of a plurality of optical sensors located at strategic locations on the structure which is being monitored, in those instances where the direction of motion of the sensor is unambiguous. Each sensor has a xe2x80x9conxe2x80x9d, or reflecting condition and a xe2x80x9coffxe2x80x9d, or non-reflecting condition. Each light pulse from the laser precedes to the optical cables via the coupler to each of the sensors in the system.
If a sensor is in its reflective condition, some tangible portion of the light pulse will travel back down the same optical fiber and pass through the Y-coupler and on to the photo detector via a cable.
The circuitry of the photo detector is programmed to clock the arrival, or non-arrival, depending on its sensors condition, and certain time windows. These are known and programmed into the computer which will therefore know which sensor is responding in whatever mode, reflected (logical 1), or non-reflective (logical 0). Because the laser is pulsing at a frequency of up to one-half a million cycles per second, 0.5 mhz, there is ample opportunity to capture the change from detectible signal to non-detectible signal without missing a step in the sequence.
Each optical sensor is mounted on a structure to be monitored to detect the relative movement of a first element of the structure relative to a second element of the structure along a first axis. Each sensor comprises a probe which is slidably mounted within a housing. The probe contains a transmissive grid, or reticle. The housing contains a reflective grid, or mask. The reticle moves longitudinally relative to the mask as the probe moves relative to the housing. An optical fiber from the fiber-optic cable extends into the housing so that the end of the optical fiber is at the reticle for transmitting a pulse of light at a right angle to the reticle. Light passing through the transmissive areas of the reticle is reflected by the mask back to the end of the optical fiber. Such a sensor is known as a reflective optical sensor. The invention disclosed in my co-pending application is also applicable to a transmissive optical sensor which is similar to a reflective optical sensor except that the reflective areas of the mask are transmissive areas. Light from the optical fiber passes through the transmissive areas of the reticle and mask and strikes the end of a second optical fiber at the opposite side of the housing for transmission to the Y-coupler. The probe is fixed to a first element of the structure to be monitored. The housing is fixed to the second element of the structure to be monitored. This system is equally applicable to rotary encoder construction with similar reflector geometry.
The reticle and mask are located in separate spaced parallel planes. The mask is mounted in the encoder for moving relative to the reticle in accordance with the relative movement between the first and second elements of the structure to be monitored. The mask and the reticle function as an encoder so that the light pulses received from the laser are reflective to the photo detector. The reticle has a plurality of evenly spaced light impervious surfaces. The areas between the light impervious surfaces are pervious to light. The pervious areas are the active areas of the reticle and the light impervious areas are the passive areas of the reticle. The mask has a plurality of evenly spaced uniform reflective surfaces which are considered the active areas of the mask. The areas between the reflective surfaces are non-reflective and are considered the passive areas of the mask.
The system of my co-pending application also includes use of xe2x80x9cquadraturexe2x80x9d, which allows the direction circuitry to be able to determine the direction of relative movement of the elements of the structure which are being monitored.
My previously disclosed pending PCT patent application describes an incremental encoder with very high resolution but which employs only three optical fibers. The centrally disposed fiber in the linear array delivers light, while the two extreme fibers carry reflected light back to their respective light detectors. The unique placement of the return fibers allows quadrature for directional determination. However, there is no present means for determining the baseline calibration once the power has been turned off.
One of the objects of the present invention is to provide a modification of the basic design of my pending PCT application such that the positive features are retained and so that remote and automatic recalibration is also possible.
Another object of the present invention is the provision of a corrosion monitoring system which is sensitive to small corrosion changes in corrodible materials being monitored.
A further object of the present invention is the provision of a corrosion monitor which is versatile for monitoring a variety of substances, is easily installed, relatively simple in construction and operation and reliable for an extended period of use.
In the monitoring system of the present application, two fibers are used for each encoder. Instead of using two fibers per sensor and making the light pulse go up and back, each of them independently, two fibers are placed in one connector and excited alternately by a pulse of light. In this way, the fiber which is not emitting a light pulse can receive it from the other and vice versa. This makes the system a continuous loop rather than an up and back along the same fiber. This is achieved by having two identical harnesses of fibers rather than having one harness longer than the other and equipping each harness with its own laser and photo diode. This improvement produces a very significant advantage. Back reflection is no longer a problem because the signal which is to be detected comes from the harness which is not pulsing so that there are zero cross-talk considerations. Also, since the time delay line is no longer needed, the sensors can be spaced much closer together for the same delay. Quadrature adjustment can be achieved by rotating the two fiber connector until the fiber offset precisely matches one-quarter of the mirror spacing. In the preferred embodiment, the mirror portion of the encoder is made essentially from a threaded rod sliding in a tube. The tops of the thread are polished down to the point where these mirrors are spaced at the desired two-to-one ratio. The fiber cores themselves constitute the other grid for most applications, making the system very simple and cost-effective.